Among the most ancient of the Olvar that lived within Arda were the trees of the Great Forests that came from the seeds that Yavanna devised during the Age of the Lamps. For many ages they grew peacefully, but in Middle-earth at the beginning of the Ages of the Stars there came among the trees great spirits later known as Ents. These protectors appeared because many other races in that time came into the world and Yavanna feared that the forests would be destroyed. So through the Ages of the Stars and Sun, the Ents walked the forests, and in time it is said some Ents became more tree-like and the ancient trees became more Ent-like and limb-lithe. Like Ents, they learned the Art of Speech. Whether tree or Ent in the beginning, by the Third Age of the Sun they were a race apart, referred to as Huorns.

The Huorns mostly stood as dark trees in the deepest forests, gnarled and unmoving, yet watchful. When aroused in wrath they moved swiftly as if wrapped in shadows, falling on foes with deadly and merciless strength.

The tale of the War of the Ring tells of how, with the Ents, the Huorns marched on Isengard like a great forest, and how, under the direction of the Ents, they exterminated the entire Orc legion in the Battle of the Hornburg.

Huorns were ancient and long-brooding, and some were said to be black-hearted and rotten. At least one such sentient tree spirit inhabited the Old Forest by the banks of the Withywindle River, who was called Old Man Willow. As the Old Forest and Fangorn were the last remnants of the most ancient forests in Middle-earth, Old Man Willow wished to prevent further inroads into his realm. He held all the Old Forest in an enchantment by the power of his song and led all travellers to him, where with his limb-lithe roots and branches he consumed them.

In The Two Towers, the Huorns are witnessed to march to Isengard with the Ents. After Isengard is harrowed of its orcs, most of the Huorns chased and killed them after the Battle of the Hornburg. After this brief appearance, the Huorns went back into the forests. The tactics used by the Huorns were quite effective, as they only attacked when all the enemy Orcs would be in their midst, denying the orcs chance of escape. It is notable that Huorns seemed to have the ability to cloak themselves in shadows, becoming almost undetectable at night.[citation needed]